Transposition immunity is a relatively unstudied genetic phenomenon in bacteria. Tn3 transposition immunity is a cis acting mechanism which limits Tn3 insertion into a plasmid. We have shown that Tn3's terminal repeat can confer immunity to a plasmid. However, we also find that the transposition immunity phenotype can be affected by host sequences. During this granting period we will pursue a genetic approach, using techniques of in vitro and in vivo mutagenesis, to dissect the immunity sequence as well as host and plasmid sequences which also affect immunity. We will determine whether other transposons related to Tn3 show immunity and if so determine whether they show cross immunity to each other. We will determine to how large a DNA molecule Tn3 can confer immunity and whether there are other parameters which also affect immunity. For example, some naturally occurring plasmids are immune to Tn3 though they do not confer penicillin resistance. Through this genetic approach, as well as additional experiments, we will distinguish between the various models for how transposition immunity works.